1. Field of the Invention
This invention relates to microchannel plate technology, and more particularly relates to an improved microchannel plate and the method for manufacturing it.
2. Description of Related Art
Microchannel Plates
The prior art microchannel plate (MCP) is used as a secondary electron multiplier consisting of millions of glass microchannels in the form of capillary tubes which are assembled and fused together to form a two-dimensional array in the shape of a disk. The capillary tubes are formed by drawing down glass-filled, glass-jacketed rods, and then etching out the glass filling. Typical microchannel diameters range from .about.40 .mu.m to .about.10 .mu.m, with the corresponding channel pitch being such that the channel cross-sectional areas constitute .about.50% of the total MCP face area. Metal films, deposited on both faces of the disk, serve as electrodes for applying an electric field across each channel and also electrically connect the multitude of channels together in parallel. Each channel behaves as a sort of continuous-dynode electron multiplier. The input end of an MCP-based detector includes a suitable photocathode optimized for the spectral characteristics of the incident radiation. The photocathode receives the incident photons and generates the primary photoelectrons which then enter the glass capillary channels. The capillary material is specially chosen (the most common being a lead-oxide glass) such that when electrons impinge on the channel walls, secondary electrons are generated. These secondary electrons are accelerated by the voltage applied across the electrodes and travel in a parabolic trajectory along the length of the channels, until, due to the transverse component of their motion, they collide with the channel walls and dislodge additional secondary electrons with each impact, thus producing electron multiplication, or gain.
In addition to the channels for secondary-electron multiplication, the photocathode for generating the primary electrons, and the front and rear electrodes for applying the accelerating electric field, a microchannel plate device generally also has an ion-barrier film at its input end. Ions may be generated within the channels by several mechanisms, including ionization of the residual gas atoms and molecules (since the vacuum in the channels is no better than .about.10.sup.-6 -10.sup.-5 Torr) and direct sputtering from the channel walls by high-energy electrons as well as other ions. The generated ions, which are accelerated toward the input face of the MCP, are prevented by the ion-barrier film from impinging upon the photocathode, which would otherwise be contaminated. The ion-barrier film, e.g., a few hundred .ANG.ngstrom thick SiO.sub.2 film, is applied as a membrane on top of the input electrode metal film.
Detection of low-level signals--optical (infrared, visible, ultraviolet and X-ray) as well as particle (electrons and ions)--is a critical requirement in a wide variety of applications, both military and civilian. A good example of devices whose performance depends on their ability to amplify very low-level input signals with large gain (.gtoreq.10.sup.4) are night vision systems, which constitute an important part of the increasingly complex and technologically-intensive equipment employed in modern warfare. Currently available high-gain detectors include numerous types of photomultiplier tubes (PMTs) and image-intensifier tubes (IITs), many of which incorporate microchannel plates as the primary amplifying device. MCP-PMTs and MCP-IITs are also extensively used in other military and space applications, such as laser satellite ranging systems, grazing-incidence telescopes for soft X-ray astronomy, and concave grating spectrometers for exploration of planetary atmospheres. Diverse scientific and technological endeavors in which MCP-based detectors are used include quantum position detectors, X-ray image amplifiers, field-ion microscopes, electron microscopes, fast oscilloscopes, observation and spectroscopy of low-level fluorescence and luminescence in living cells, radioluminescence imaging, and time-correlated photon counting. Indeed, the technology for manufacturing instruments for capturing and displaying images at low light levels has progressed to such a level that several new areas of investigation, some mentioned above, which were previously thought inaccessible, have now opened up.
Quantitative detection and imaging of objects at low light levels places demanding requirements on the performance parameters of the detectors relating to spatial resolution, signal-to-noise ratio, response speed, output linearity, dynamic range, and reliability. Although currently available detectors perform satisfactorily in several applications, significant further improvements in resolution are desirable and necessary to extend the applicability of MCP-based devices to meet more critical requirements. Even though MCP channel diameters as small as 5 .mu.m are available, the resolution degrades significantly because of the extremely low Modulation Transfer Function (MTF). This results from the spreading of the secondary electron beam to a width several times the channel diameter. There is, therefore, a need for means to focus the individual beams to increase the MTF. This focusing need cannot be met by present MCP manufacturing technology. Simultaneously, the complexity of present manufacturing technologies and the extremely limited number of manufacturers have made the cost of such detectors too prohibitive to allow their use in a wider scope of applications that require low-cost equipment, such as inexpensive night-vision goggles, fast (&gt;1 GHz) oscilloscopes, and optical computing.
Microchannel plates are almost always geometrically so designed that the channel axes are at a small angle (`bias angle`) to the perpendicular to the input and output faces. In addition, often two MCPs are used in tandem in a chevron geometry. The bias angle and the chevron geometry not only insure that the primary photoelectron emitted by the photocathode will strike the channel wall near the channel's entrance, but also reduce the light feedback from the phosphor screen at the output end of the MCP to the photocathode. Additionally, the bias angle also contributes in reduction of the ion feedback.